The production of sodium hydroxide and chloride from an aqueous solution of alkali chloride, especially brine, by electrolysis has long been one branch of the basic chemical industries. Initially, electrolysis was carried out by using mercury as the cathode to yield alkali hydroxide and chlorine of extremely high purity. Use of the mercury method, however, is diminishing because of high energy consumption (approximately 3000 kWh/ton of alkali hydroxide) and environmental pollution with mercury. As a substitute for the mercury method, a new method has been developed which uses an asbestos diaphragm. This new method suffers from the disadvantages of forming alkali hydroxide of low purity, requiring an additional step for separating alkali hydroxide from alkali chloride, and permitting a large amount of oxygen to enter chlorine. Its advantage of low energy consumption for electrolysis is offset by high energy consumption for product purification. The overall energy consumption is equal to or more than that of the mercury method. Another disadvantage is that asbestos is a carcinogen. As a result, the ion exchange membrane method is becoming predominant in the field of alkali chloride electrolysis.
The ion exchange membrane method is designed such that a purified aqueous solution of alkali chloride (especially sodium chloride) is fed into the anode compartment (which is separated by a cation exchange membrane from the cathode compartment in the electrolytic cell) and pure water is fed into the cathode compartment as needed so as to yield chlorine in the anode compartment and alkali hydroxide (30-50%) in the cathode compartment. The energy consumption of this method is 2200-2500 kWh/ton of alkali hydroxide, which is 20-30% less than that of the other conventional method. In Japan, for example, the production of more than 80% of alkali hydroxide is by the ion exchange membrane method.
Despite its advantages, the ion exchange membrane method has a disadvantage in that up to ten percent of the alkali hydroxide formed in the cathode compartment migrates into the anode compartment through the ion exchange membrane. The ratio of the amount of alkali hydroxide excluding the migrated alkali hydroxide to the total amount of alkali hydroxide is expressed by the term of current efficiency. It is usually 90-97%, depending on the kind of the ion exchange membrane used. Not only does the migrated alkali hydroxide decrease the current efficiency in proportion to its amount, it also reacts with chlorine in the anode compartment to form chloric acid and chlorate. The major constituent of the chlorate is sodium chlorate, which is extremely stable and hardly decomposes. The accumulation of sodium chlorate decreases the solubility of alkali chloride in its aqueous solution. The decreased concentration of alkali chloride permits more oxygen to enter chlorine formed in the anode. This has an adverse effect on electrolysis itself.
This disadvantage can be eliminated by adding hydrochloric acid to the anode compartment in an amount equivalent to the current efficiency of the ion exchange membrane. The hydrochloric acid neutralizes the alkali hydroxide which has migrated from the cathode compartment through the cation exchange membrane, thereby converting the alkali hydroxide into the initial alkali chloride in the anode compartment. This prevents the adverse effect caused by the formation of sodium chlorate, and acidifies the anode compartment, which leads to improved purity of chlorine obtained.
However, the addition of hydrochloric acid poses a problem associated with the uneven distribution of acid concentration in the electrolytic cell. If the addition method is not precise, it may cause local corrosion in the various parts of the electrolytic cell. Further, it is necessary to use synthetic hydrochloric acid of high purity. In other words, it is necessary to produce hydrochloric acid from chlorine obtained by electrolysis. This lowers the efficiency of chlorine production and adds the cost for synthesis of hydrochloric acid from chlorine to the cost of the process.